Without DEBIAN_FRONTEND set to noninteractive, any apt-install might show an
interactive dialog and hence hangs.
Fixes output like this:
```
Setting up tzdata (2020a-0ubuntu0.20.04) ...
debconf: unable to initialize frontend: Dialog
debconf: (TERM is not set, so the dialog frontend is not usable.)
debconf: falling back to frontend: Readline
Configuring tzdata
------------------
Please select the geographic area in which you live. Subsequent configuration
questions will narrow this down by presenting a list of cities, representing
the time zones in which they are located.
1. Africa 4. Australia 7. Atlantic 10. Pacific 13. Etc
2. America 5. Arctic 8. Europe 11. SystemV
3. Antarctica 6. Asia 9. Indian 12. US
```
Signed-off-by: Armin Friedl <dev@friedl.net>
